The invention relates to a focus state detection device used in a camera or the like.
Conventionally, a method that has been known for focus state detection in an automatic focus state detection device of a camera is the phase-difference detection method, wherein two images having a parallax of the subject are guided to a pair of image sensors, the focus state being determined by computing the relative amount of divergence between the two images from the images output from the pair of image sensors.
This phase difference detection method will be explained with reference to FIG. 4.
Light rays which are incident via region 21 of a shooting lens 1 are focussed on the film equivalent plane 6. The light rays then pass through a focus state detection optical system 8 comprised of a band-pass filter 7, a field mask 2, a field lens 3, an aperture stop 41 and a re-imaging lens 51, and are composed into an image on a sensor array 9A of an image sensor 9. Similarly, light rays which are incident via a region 31 of the shooting lens 1 are focussed on the film equivalent plane 6. The light rays then pass through a focus state detection optical system 8 comprised of a band-pass filter 7, a field mask 2, a field lens 3, an aperture stop 42 and a re-imaging lens 52, and are composed into an image on a sensor array 9B of the image sensor 9.
The size of the region 21 in shooting lens 1 is the same as the inverted projection of the aperture stop 41 through the field lens 3 and, similarly, the size of the region 31 in shooting lens 1 is the same as the inverted projection of the aperture stop 42 through the field lens 3.
The pair of subject images composed on sensor arrays 9A,9B of the image sensor 9 by the focus state detection optical system 8 become mutually farther apart in a so-called front focus state with a clear image of the subject being composed in front of the predicted focus plane by the shooting lens 1 and, conversely, become mutually closer together in a so-called rear focus state with a clear image of the subject being composed behind the predicted focus plane 6.
At the so-called in-focus time, when a clear image of the subject is composed exactly at the predicted focus plane by the shooting lens 1, the subject images on the image sensor arrays 9A and 9B mutually coincide.
Accordingly, an automatic focus state detection device of a phase difference method photoelectrically converts the pair of secondary images of the subject image composed by the focus state detection optical system 8 into electrical signals using the sensor arrays 9A,9b of the image sensor 9. Further, the focus adjustment state of shooting lens 1, herein being the amount of separation from the focus state and the direction of such (hereafter called the defocus amount) can be determined by performing focus state detection correlation algorithm processes on the pair of subject image signals and by finding the relative position of the secondary images of the pair of subject images.
In addition, the subject image signals obtained from the image sensor 9 through photoelectric conversion have differing voltages after photoelectric conversion depending upon the luminosity of the subject. Consequently, it is necessary to control the electric charge accumulation time so that the subject image signal is of a level suitable for focus state detection algorithms, regardless of the subject.
For example, one method of controlling the electric charge accumulation time is known wherein the electric charge accumulation time is controlled so that the peak value of the subject image signal is a predetermined value. Calling Tp the accumulation time of the current accumulation, Pp the peak value of the subject image signal and Pk a predetermined value, the next accumulation time Tq that is to be controlled can be determined from equation 1: EQU Tq=Tp*Pk/Pp. (1)
The peak value Pq of the subject image signal accumulated next time with accumulation time Tq is the same as the predetermined value Pk.
However, the following problems arise in electric charge accumulation control in the conventional type of focus state detection device described above.
For example, when the accumulation time is controlled relative to the first subject captured in the focus state detection area so that the peak value of the subject image signal becomes the predetermined value, and after the shooting lens has focussed on the subject which is the first captured, a bright subject enters into the focus state detection area, accumulation is controlled so that the peak value of the subject image signal becomes the predetermined value and, consequently, the peak value of the subject image signal for the entire focus state detection area comes from the portion of the subject image signal received from the high luminosity object, the peak value of the subject image signal of the high luminosity object being controlled so as to be at a suitable level. Consequently, the level of the subject image signal of the subject captured initially in the focus state detection area drops and becomes a value that is not suitable for focus state detection algorithms so that, in essence, focussing is effected relative to the high luminosity object.
On the other hand, when the accumulation time is controlled initially relative to a high luminosity subject so that the peak value of the subject image signal becomes a predetermined value, and after focussing has been effected relative to the initial high luminosity subject, the high luminosity subject leaves and only dark subjects, such as the background or the like, are captured in the focus state detection area, accumulation is controlled so that the peak value of the subject image signal of the dark subject becomes the predetermined value, the accumulation time becoming longer than the accumulation time relative to the high luminosity subject. However, because the accumulation time is controlled so that the peak value of the dark subject becomes the predetermined value, immediately after the high luminosity subject is recaptured in the focus state detection area, the subject image signal of the high luminosity subject is saturated. In addition, in accumulations following this, control is such that this saturation level is considered to be the peak value and, consequently, particularly when the difference in luminosity between the high luminosity subject and the dark subject is large, considerable time is required for the peak value of the subject image signal to reach a suitable level.
Another problem occurs when a light, such as a fluorescent light, is repeatedly turned on and off at a periodic rate, that is, it flickers, which cannot be detected by the naked eye, with electric charge accumulation control there are cases wherein accumulation is conducted in a shorter length of time than a single turning on and off, or flicker, of the fluorescent light. Accordingly, with subjects under fluorescent light, the level of the subject image signal will vary greatly with the timing of accumulation relative to the turning on and off of the fluorescent light when the accumulation time is shorter than a single turning on and off of the fluorescent light.